1. Field of the Invention
The invention relates to a method of fabricating a ferromagnetic tunnel junction device, and more particularly to a method of a ferromagnetic tunnel junction device used for a sensor for reading a magnetic head in an apparatus for reading a high density magnetic disc.
2. Description of the Related Art
Some of conventional ferromagnetic tunnel junction devices are designed to include a pair of ferromagnetic layers, and a tunnel barrier layer sandwiched between the ferromagnetic layers and composed of an electrical insulator with a few nanometer thickness. When an external magnetic field is applied to the ferromagnetic layers with a constant current being applied across the ferromagnetic layers, there appears magneto-resistance effect where a resistance varies in dependence on a relative angle formed between orientations of magnetization of the ferromagnetic layers.
When the orientations of magnetization are parallel with each other, a resistance is minimum, and when the orientations of magnetization are not parallel with each other, a resistance is maximum. Hence, it is possible to make the orientations of magnetization in parallel or non-parallel with each other in accordance with an intensity of an applied magnetic field by providing a difference in coercive force to the ferromagnetic layers. This means that an intensity of a magnetic field can be detected by monitoring variation in a resistance.
Recently, there has been obtained a ferromagnetic tunnel junction device which includes a tunnel barrier layer composed of a film obtained by oxidizing a surface of an aluminum layer, to thereby provide about 20% of a magneto-resistance ratio. This ferromagnetic tunnel junction device is able to be applied to a magnetic head, a magnetic memory, and so on. Such a ferromagnetic tunnel junction device with a high magneto-resistance ratio is suggested, for instance, in Journal of Applied Physics, Vol. 79, April 1996, pp. 4724-4729.
In fabrication of the above-mentioned ferromagnetic tunnel junction device with a high magneto-resistance ratio, a first ferromagnetic layer composed of CoFe is deposited onto a glass substrate by means of vacuum evaporation in which an evaporation mask is employed, and then, an another evaporation mask is employed to thereby form an aluminum layer on the first ferromagnetic layer by evaporation by a thickness in the range of about 1.2 nm to 2.0 nm. Then, the thus formed aluminum layer is exposed to oxygen glow discharge to thereby form a tunnel barrier layer composed of aluminum oxide (Al.sub.2 O.sub.3) on the aluminum layer. Then, a second ferromagnetic layer composed of Co is formed on the tunnel barrier layer so that the second ferromagnetic layer overlaps the first ferromagnetic layer through the tunnel barrier layer.
Thus, there is completed a cross-electrode type ferromagnetic tunnel junction device. This device provides about 18% of a magneto-resistance ratio at maximum.
Apart from the above-mentioned Journal of Applied Physics, a ferromagnetic tunnel junction device has been suggested in many documents such as Japanese Unexamined Patent Publications Nos. 5-63254, 6-244477, 8-70148, 8-70149, 8-316548, and 9-106514, and Journal of Japan Applied Magnetic Society, Vol. 21, 1997, pp. 493-496. These Publications and Journal have suggested a method of forming a tunnel barrier layer, comprising the steps of forming an aluminum layer, and exposing the aluminum layer to atmosphere to thereby grow an aluminum oxide (Al.sub.2 O.sub.3) layer.
In an application of a ferromagnetic tunnel junction device to a magnetic head used in an apparatus for reading a high density magnetic disc, it is necessary to reduce a resistance to some degree without increasing a size of the device, in order to minimize thermal noise. To this end, a ferromagnetic tunnel junction device is usually polished at its end surface to thereby define a height of the device with high accuracy, which is perpendicular to a width of the device, ensuring reduction in a resistance.
In an application of a ferromagnetic tunnel junction device to a magnetic head, a surface of a ferromagnetic tunnel junction device faces a recording surface of a magnetic disc, and an intensity of a magnetic field is detected.
In a conventional method of polishing a surface of a ferromagnetic tunnel junction device, a tunnel junction surface is polished on a polishing plate adding dropwise of water or oil including hard diamond particles with a few micrometer in diameter. The surface is mechanically polished by virtue of polishing function and plastic flow function of the hard particles, to thereby flatten.
It is necessary for first and second ferromagnetic layers in a ferromagnetic tunnel junction device to be electrically isolated from each other by a tunnel barrier layer sandwiched therebetween and having a thickness of a couple of nanometers.
However, when a ferromagnetic tunnel junction device is mechanically polished, the first and second ferromagnetic layers are sometimes locally short-circuited with each other by the above-mentioned plastic flow function of hard particles. In other words, since the tunnel barrier layer is quite thin, specifically, has a thickness of a couple of nanometers or smaller, if the first ferromagnetic layer is plastically deformed and fluidized in a polishing direction in the step of mechanically polishing a ferromagnetic tunnel junction device, electrons sometimes pass through the tunnel barrier layer and reach the second ferromagnetic layer from the first ferromagnetic layer.
If the first and second ferromagnetic layers are short-circuited with each other, namely, make direct contact with each other, it would be impossible to sufficiently have the tunnel effect which takes place through the tunnel barrier layer, resulting in deterioration in device performances.